valid_shadow - controls which objects can be shadowed
int valid_shadow( object ob );
When an object attempts to shadow 'ob' (with the shadow() efun),
valid_shadow in the master object is called. One object parameter is
passed, which is the object that previous_object() is attempting to
shadow. valid_shadow() should return 0 if the shadow should not be
permitted, in which case the shadow() call will return 0 and fail. If
valid_shadow() returns 1, the shadow is allowed.